In conventional lighting applications, incandescent lamps are used, which are based on resistive heating of an light-emitting element. The lamps are powered from mains, which in Europe involves 230 V AC at 50 Hz. Conventionally, such lamps are connected in series with an ON/OFF switch, so that the lamps are either off or produce light at a nominal power. To meet the demand of being able to dim the lamp, i.e. to reduce the light level, mains dimmers have been developed as a device being connected in series with the lamp, either as a separate device or integrated with the wall-mount switch. Such dimmers typically operate on the basis of phase-cutting the mains voltage, using a TRIAC. Since such dimmers are commonly known, the following description will be kept brief.
FIG. 1A is a graph schematically showing voltage (vertical axis) versus time (horizontal axis) of rectified mains. It can be seen that this voltage follows a continuous sine-shaped curve, of which the negative portions are inverted. The power provided to a resistive load, expressed as P=U2/R, can be considered as being proportional to the surface area under the curve.
FIGS. 1B and 1C are comparable graphs showing the output voltage of phase-cutting dimmers, i.e. a leading edge dimmer (FIG. 1B) or a trailing edge dimmer (FIG. 1C). In the case of a leading edge dimmer (FIG. 1B), the output voltage is suppressed to remain zero immediately after a zero-crossing of the mains, until a certain phase pX between 0 and 180° when the voltage makes a jump to follow the mains curve. Again, the power provided to a resistive load can be considered as being proportional to the surface area under the curve: it can be seen that this power is reduced when said phase pX is increased (righthand side of the curve). In the case of a trailing edge dimmer (FIG. 1C), the voltage follows the mains after a zero-crossing until a certain phase pY between 0 and 180° when the voltage is suppressed to make a jump to zero. Again, the power provided to a resistive load can be considered as being proportional to the surface area under the curve: it can be seen that this power is reduced when said phase pY is decreased (lefthand side of the curve). The situation of FIG. 1B is indicated as “leading edge dimming” and the situation of FIG. 1C is indicated as “trailing edge dimming”.
One problem with incandescent lamps is their low efficiency: much of their power consumption goes wasted in the form of heat. Alternative light sources have been developed with much higher efficiency, such as for instance gas discharge lamps, LEDs, OLEDs, etc. For producing the same amount of light, such light sources require much less electrical energy, for which reason they can be termed “energy-saving lamps”. Such light sources can not be operated by direct connection to mains: they need to be driven by a driver device which in turn is powered from mains. Drivers include the CuFe ballast for TL lamps, but the present invention relates to electronic drivers. As will be commonly known to persons skilled in the art, such electronic drivers have been developed for on the one hand presenting a suitable load to mains (power factor correction) and on the other hand generating a suitable output current for the light source. The driver may be designed for controlling the current magnitude, but may also be designed for controlling the output power.
It is also desirable to be able to dim an energy-saving lamp, so dimmable electronic drivers have also been developed. Such driver has a user control input, typically wirelessly coupled to a remote control, via which a user may control the light intensity, i.e. dim the light source. In such case, dimming is performed by the driver, by reducing the output current intensity (amplitude) or reducing the PWM duty cycle of the lamp current.
“Normal” dimmable electronic drivers are intended for being powered from “normal” mains, i.e. a sine-shaped voltage of for instance 230 V 50 Hz in Europe. However, there are also situations where it is desirable to be able to dim the lamp by using a mains dimmer. Such situation may typically occur when an existing light source is replaced by a light source with integrated electronic driver, when the existing light source is for instance an incandescent lamp powered via a wall-mounted dimmer. Electronic drivers capable of receiving “dimmed” mains from a mains dimmer have also been developed: such drivers can be described as operating as normal drivers as far as generating current for the lamp is concerned, yet having the further facility of analysing the phase angle of the input mains and determining the dim level for the lamp on the basis of this information.
FIG. 2 is a block diagram of a practical situation where an electronic lamp driver 20 is connected to “dimmed mains” provided by a mains dimmer 10 operating according to the above-described phase-cutting principle. The dimmer 10 has an input 11 receiving the original mains, and an output 12 providing dimmed mains VD. The mains dimmer 10 has a user input 13 for controlling the dim level, typically a rotating knob. In a common situation, the mains dimmer 10 is wall-mounted while a lamp L supplied by the mains dimmer 10 is mounted relatively remote, illustrated by the long supply lines 14. The lamp L is provided with an electronic driver 20, either as a separate device or as a built-in device, having an input 21 connected to the supply lines 14 to receive the dimmed mains VD, and having an output 22 connected to the actual light source of the lamp L.